1. Field of the Invention
The present invention relates to an ultrahigh-frequency electronic component comprising an ultrahigh-frequency whip encased in a molded resin and a method of manufacturing such an ultrahigh-frequency electronic component.
2. Description of the Prior Art
Ultrahigh-frequency electronic devices include transistors, ICs, optical devices, surface-acoustic-wave devices, and resonators. Since these devices are used in commercial communication apparatus and satellite-mounted apparatus, they are required to be highly reliable and to have a long service life. Therefore, these ultrahigh-frequency electronic devices are assembled in hollow packages of ceramics.
In recent years, many ultrahigh-frequency electronic devices are finding use in home electronic appliances such as satellite broadcast receivers, portable telephone sets, etc. Since devices for use in home electronic appliances do not need to be so reliable and durable as those in commercial communication apparatus and satellite-mounted apparatus, it is desirable that they can be manufactured less expensively. One approach to reduce the cost of ultrahigh-frequency electronic devices for home use is to encase them in a molded resin packages according to a transfer molding method.
The molded-resin packages have the same structure as those of semiconductor devices. Specifically, an ultrahigh-frequency chip is mounted on a lead frame and then electrically connected thereto by wire bonding, and the entire assembly is placed in a mold die set where it is encased in a molded epoxy resin.
When the ultrahigh-frequency chip and the lead frame are encased in their entirety in the molded epoxy resin, since the epoxy resin has a relatively large permittivity, large parasitic capacitances are developed between conductors on the chip or bonding wires, preventing the ultrahigh-frequency chip from having a desired high-frequency gain.
In order to alleviate the above drawback, it has been customary to use a resin having a relatively small permittivity to encase ultrahigh-frequency chips, to reduce the permittivity of a portion of the resin mass which covers the ultrahigh-frequency chip, or to provide a hollow in the molded-resin package for exposing the ultrahigh-frequency chip.
One molded-resin package which uses a resin having a relatively small permittivity is disclosed in Japanese Patent Laid-open Publication No. 86-237455, for example.
The disclosed molded-resin package comprises an ultrahigh-frequency semiconductor chip covered with a fluoroplastic sealing resin having a low permittivity, and the molded sealing resin is further encased in an epoxy resin. However, the disclosed molded-resin package is expensive to manufacture because the fluoroplastic resin is used as the sealing resin having a low permittivity. Another problem of the disclosed molded-resin package is that the fabrication process control is complex because it uses two types of resin.
Japanese Patent Laid-open Publication No. 88-79353, for example, shows a molded-resin package in which the permittivity of a portion of the resin mass which covers an ultrahigh-frequency semiconductor chip is relatively small. The disclosed molded-resin package is fabricated by supplying an ultrahigh-frequency semiconductor chip with a first sealing resin which comprises an epoxy resin mixed with minute hollow bodies to seal the surface of the ultrahigh-frequency semiconductor chip according to a potting method, and then encasing the first sealing resin with a second sealing resin according to a transfer molding method. The minute hollow bodies are in the form of hollow glass balls.
However, because the minute hollow bodies are mixed with the first sealing resin in order to reduce the permittivity thereof, the cost of the molded-resin package increases by the cost of the minute hollow bodies. The process of sealing the surface of the ultrahigh-frequency semiconductor chip with the minute hollow bodies comprises a relatively large number of steps as it is necessary to disperse the minute hollow bodies uniformly in the first sealing resin.
A hollow molded-resin package is revealed in Japanese Patent Laid-open Publication No. 86-166138, for example. According to the disclosed fabrication process, a foaming agent is introduced into a sealing resin to cover an ultrahigh-frequency semiconductor chip, and after the sealing resin is poured into a mold die set, the foaming agent is foamed to produce a hollow where the ultrahigh-frequency semiconductor chip is exposed.
The cost of the hollow molded-resin package thus manufactured increases by the cost of the foaming agent. In addition, since the size and shape of the hollow cannot be controlled, ultrahigh-frequency semiconductor devices in such hollow molded-resin packages tend to suffer electric characteristic variations. If the hollow is excessively large, then the wall thickness of the molded sealing resin is excessively small, resulting in a reduction in the rigidity of the hollow molded-resin package.